This invention generally relates to probes for inspecting tubes, and is specifically concerned with a flexible eddy current probe capable of scanningly inspecting the interior wall of a heat exchanger tube in a nuclear steam generator.
Probes for inspecting the interior walls of heat exchanger tubes are known in the prior art. Such probes are particular useful in determining whether or not flaws such as cracks, pits, or localized areas of wall thinning are present in the heat exchanger tubes of operational nuclear steam generators. The ability of such probes to determine whether or not such flaws may be present is of paramount importance, as such flaws all constitute potential leak sites which could allow the radioactive primary side water which flows within these tubes to leak out into the non-radioactive, secondary water which surrounds these tubes and which is used to create the steam that drives the turbines in the plant. It is also important that the probe succeed in determining not only the presence of such a flaw, but also its location along the longitudinal axis of the tube so that a repair operation (such as a sleeving operation which creates an internal hydraulic "bridge" across the flawed section of the tube) may be performed in order to prevent radioactive water from the primary side of the generator from contaminating the secondary water in the generator. Finally, as the heat exchanger tubes in many steam generators are characterized by a bent portion in their center sections (called a "U-bend" in the art), it is further important the probe be flexible so that it can readily traverse and inspect the U-bend of the tube (which is, incidentally, one of the most common situses for tube flaws).
FIG. 1 illustrates one of the most advanced, state-of-the art probe designs for fulfilling all the aforementioned criteria.
One of the most advanced probes for inspecting such heat exchanger tubes generally comprises a probe head assembly that is connected to a flexible cable which is capable of both axially moving the head assembly to a desired location along the longitudinal axis of a heat exchanger tube, as well as rotating it so that a small "pancake-type" eddy current coil located on the side of the probe head assembly scanningly inspects the inner wall of the tube. To this end, the probe head assembly includes a generally egg-shaped body member, and a sensor holding mechanism for holding the eddy current probe and radially biasing it toward the inner wall of the tube. Such probes further include a pair of centering mechanisms which are disposed around the flexible cable that both enters into and exits from the body member of the probe head assembly which are adjacent to both the bottom and the top ends of the probe head assembly. The purpose of the centering mechanisms is to maintain the flexible cable in alignment with the center line of the tube when the cable is used to rotate the probe head assembly. To achieve this purpose, each of the centering mechanisms includes an annular member whose interior is fixedly engaged around the flexible cable, and whose exterior holds a plurality of radially disposed, monofilament fingers for bristles which sweepingly engage the inner walls of the tube when the probe is either moved axially or rotated within the tube.
While such probes are generally capable of achieving their intended purpose, the applicants have noted a number of drawbacks associated with this particular probe design. For example, the applicants have noted that the flexing of the bristles that radially extend out of the centering mechanisms can impart a jerky movement of the probe along the longitudinal axis of the heat exchanger being inspected. Such jerkiness in turn impairs the quality of the information received by the eddy current probe on the probe head assembly. Still another drawback associated with this design is the occasional failure of the probe head assembly to maintain its axis of rotation in alignment with the center line of the tube during a scanning operation. The resulting chattering of the head assembly against the inner walls of the tube further impairs the quality of the information received from the probe. A third drawback associated with this design is the fact that unwanted probe "lift-off" can occur when the probe head assembly is rotated throughout a section of the tube which has been rendered slightly ovalur by, for example, the mechanical process which create a U-bend in the center portion of such heat exchanger tubes. This is a particularly serious drawback, as the U-bend section of the tubes are more apt to have flaws as a result of the residual stresses created in these regions from the bending operations. The resulting probe lift-off can cause it either to fail to detect what serious flaws may be present, or to generate spurrous readings that indicate the presence of flaws that are in fact not there. Other shortcomings associated with this prior art probe include the fact that it is difficult, if not impossible to replace the delicate pancake-type eddy current probe if it should become damaged or fail. Also, this design allows the creation of a gap between the probe holding mechanism in the egg-shaped probe body which unfortunately is apt to catch the edges presented by the open ends of the heat exchanger tubes when the probe is being initially inserted into the tube.
Clearly, there is a need for a probe which retains all of the advantages associated with the previously described flexible probe, while eliminating all of the drawbacks.